The present invention relates to electrical induction logging systems and apparatus which predict the nature and characteristics of subsurface formations penetrated by a borehole. More particularly, the invention relates to induction logging tools employed in measurement-while-drilling (MWD) applications. Still more particularly, the invention relates to apparatus for sealing and physically protecting the antenna arrays and associated circuitry contained in an induction logging tool from the extreme pressures and harsh environment associated with MWD operations.
Recovery of hydrocarbons from subsurface formations typically commences by drilling a borehole through the earth to a subsurface reservoir or other location thought to contain hydrocarbons. As drilling progresses, various physical, chemical and mechanical properties are measured and "logged" for the purpose of determining the nature and characteristics of the subsurface formations, including, for example, the porosity, permeability, saturation and depth. One such logging technique commonly used in the industry is referred to as induction logging. Induction logging measures the conductivity, or its inverse, the resistivity, of the formation surrounding the borehole. Formation conductivity is one possible indicator of the presence or absence of a significant accumulation of hydrocarbons because, generally speaking, hydrocarbons are relatively poor conductors of electricity. Formation water, on the other hand, typically having a relatively high concentration of dissolved salt, is a relatively good conductor of electricity. Thus, induction logging tools can obtain information that, properly interpreted, can indicate the presence or absence of hydrocarbons.
U.S. Pat. Nos. 4,553,097, 4,659,992, 4,785,247 and Re 32,913 illustrate typical prior-art tools that operate according to the basic principles of induction logging. The downhole tool comprises at least one transmitter antenna or coil and one or more receiver coils spaced apart coaxially from the transmitter. A signal generator connected to the transmitter coil produces an alternating current within the transmitter coil. The flow of alternating current in the transmitter coil induces a magnetic field within the surrounding formation, causing eddy currents to flow circumferentially about the axis of the tool and into the formation. The eddy currents, in turn, induce a magnetic field that is coupled to the receiver coils, thereby inducing in the receiver coils a voltage signal with magnitude and phase dependent upon the electrical characteristics of the adjacent formation.
To be of use to the driller, the information received by the tool must be communicated to the surface. One prior art method of obtaining at the surface the data taken at the bottom of the borehole is to withdraw the drill string from the hole, and to lower the logging tool down the hole by means of a wire cable. Using such "wireline" apparatus, the relevant data may be transmitted to the surface via communication wires or cables that are lowered with the tool. Alternatively, the wireline logging tool may include an electronic memory such that the relevant information may be encoded in the memory to be read when the tool is subsequently raised to the surface. Among the disadvantages of these wireline methods are the considerable time, effort and expense involved in withdrawing and replacing the drill string, which may be, for example, many thousands of feet in length. Furthermore, updated information on the drilling parameters is not available while drilling is in progress when using wireline techniques.
A much-favored alternative is to position the logging tool within the drill string close to the drill bit so as to gather the formation data while drilling is in progress. Using such techniques, the transmitter and receiver coils are typically wound about a drill collar located on the lower end of the tool string. The data obtained by the tool is then transmitted to the surface by a mud pulse system of telemetry or another form of telemetry. The mud pulse system creates acoustic signals in the drilling fluid which is circulated through the drill string during drilling operations. The acquired formation data, along with data gathered by other sensors that are helpful or necessary to the driller, are transmitted by suitably timing the formation of pressure pulses in the mud stream. The pressure pulses are received and decoded by a pressure transducer and computer at the surface.
The techniques for obtaining formation data and other downhole parameters while drilling is in progress are generally termed "measurement while drilling" or MWD. MWD results in a major savings in drilling time and cost compared to the wireline methods described above, primarily because it permits important data to be transmitted up to the surface without requiring the time consuming use of a wireline tool.
In conventional MWD induction logging tools, the transmitter and receiver coils are typically mounted in a series of spaced-apart recesses that are machined into the outer surface of a drill collar. Each coil is wound about the tool within one of the recesses. The antenna wire is then typically covered and the recess filled with an insulative material, such as nitrile rubber. Such a structure is very common and is disclosed, for example, in U.S. Pat. No. 4,651,101 (FIG. 4 and accompanying text) and U.S. Pat. No. 4,785,247 (FIGS. 2A and 2B and accompanying text). U.S. Pat. No. 4,651,101 discloses another example of conventional means for mounting the receiver and transmitter coils in an induction logging tool. As shown and described with respect to FIG. 5 of that patent, the coils are wound about a central support at spaced-apart locations and are covered by a fiberglass sleeve. Pressurized oil surrounds the coils and fills the annular space between the central support and the fiberglass sleeve. Pressure compensation devices are provided to maintain the oil at a pressure higher than the pressure of the borehole fluids so that the differential pressure on the fiberglass sleeve will be small and so as to prevent the delicate antenna arrays from becoming damaged.
In addition to the antennas themselves, there are usually a variety of electronic devices that must be mounted near the antennas in order for the data retrieval system to function properly. These electronics packages usually take the form of printed circuit boards which perform certain signal conditioning functions, such as amplification or tuning. As disclosed, for example, in FIG. 27 of U.S. Pat. No. 4,949,045, the electronics are typically housed in compartments that are machined into the drill collar at locations that are spaced-apart from the recess in which its associated antenna coil is embedded. Each electronics compartment includes a cover that is meant to seal the electronics from the drilling fluids flowing in the borehole. A wireway interconnects each electronics compartment with the recess in which its associated antenna is embedded. Because the electronics must be sealed from the drilling fluids flowing in the borehole, and because the antenna recesses are not sealed from the intrusion of drilling fluids, a specialized and costly hermetic bulkhead feedthrough device must be positioned in each wireway between the electronics compartment and the antenna recess to ensure that no fluid in the antenna recesses passes into the electronics compartment.
Although MWD induction logging tools have gained great acceptance, the present tools suffer from a variety of shortcomings. First, the antenna coils are generally not well protected under present-day designs. The insulation covering the recesses in which the coils are embedded is frequently torn or punctured by the borehole when subjected to the hostile downhole environment associated with MWD applications. When the insulation is damaged or destroyed by abrasion or otherwise, the antenna themselves may be destroyed. Further, the pressure feedthroughs may also be damaged, allowing drilling fluid to escape into one or more of the electronics compartments and damaging or destroying those signal conditioning components as well. A fiberglass sleeve such as that shown in U.S. Pat. No. 4,651,101 adds little in the way of protection as it is easily punctured or otherwise damaged due to the tremendous forces imparted to the tool in operation, particularly during steerable or directional drilling operations where enormous side wall forces are imparted to the tool.
Damage to the logging tool has the obvious effect of necessitating repairs to the tool and the replacement of some or all of its internal electronics and antennas. However, the economic harm caused by a damaged MWD tool is even more substantial than the costs associated with simply making such repairs. The true economic harm can be better appreciated when it is considered that the damage and the resulting loss of signal from the MWD tool will necessitate a time consuming and extremely costly procedure where the entire drill string must be removed from the borehole, one pipe section at a time, so as to bring the damaged tool to the surface for repair or replacement. Then, after such repairs are completed, the drill string and the tool must be replaced in the hole. The high labor and equipment costs incurred by the driller during this unproductive period where no hole is being drilled can be economically disastrous.
Adding further to the harm caused by a damaged logging tool is the fact that even after a damaged tool has been removed from the hole, it is frequently very difficult to repair the tool in the field. Typically, when damage occurs so as to cause one electronics compartment to be contaminated by drilling fluids, all or at least some of the other compartments will likewise be affected. In certain of the prior art tools, each electronics compartment is covered by a separate access panel. Each panel must be removed by extracting a number of screws or similar fasteners. Removing a number of these panels, cleaning the compartments, replacing and testing the electronics, and then replacing the panels using the many individual fasteners can be a very time consuming and thus costly task.
As is apparent, then, despite the advances made in MWD and induction logging technology over the years, there remains a need within the industry for an induction logging tool that will better survive the harsh duty to which MWD tools are subjected. More specifically, improvements in the area of sealing and protecting the antenna arrays and associated electronics would be welcomed by the industry. Preferably, an MWD logging tool could be developed that would be highly resistant to the damage caused by abrasion and punctures to the insulative materials that are presently used to surround the antenna arrays on conventional tools. It would be further desirable if the tool would simply and effectively seal and protect all the antenna arrays and electronics that are housed within the tool, without reliance on a number of separate and discrete access panels. Ideally, the electronics compartments and antenna recesses of the tool would all be sealed by a single, easily removable cover, so as to eliminate the need for separating the electronics compartments from the antenna recesses. Such a feature would eliminate the need to rely on costly pressure feedthroughs for the interconnecting wiring and allow the signal conditioning electronics and antenna all to operate at atmospheric pressures.